In objects having a size of only a few nanometers, which are known as quantum dots, nanodots, or nanoislands, the freedom of motion of the electrons is restricted in all three spatial directions (“zero-dimensional system”). Thus, the linear dimension in all three directions is less than the de Broglie wavelength of the charge carriers. Such quantum dots have a greatly modified electronic structure from the corresponding bulk semiconductor material and, in particular, the density of states becomes more like that for molecules. Quantum dots have a discrete energy spectrum and, in some aspects, behave similarly to atoms, which is due to the quantum nature of the electronic structure. However, unlike with atoms, it is possible to influence the size and electronic structure. Due to the small electrical capacitance of the quantum dots, the addition of a further electron to the electrons already present in the quantum dot (“single-electron tunneling”) requires a certain amount of energy, ranging from several tens of meV to several hundreds of meV (“Coulomb blockade”). This effect allows for controlled quantization of the current flow through the quantum dot. The size and shape of the quantum dots are dependent on the production method and the elements used. At present, quantum dots are mainly used in nanooptics and nanoelectronics, for example, in photodetectors and semiconductor lasers, and also in solar cells. In particular, the formation of binary, ternary, or multinary compound semiconductor quantum structures in a semiconductor matrix is becoming increasingly important in the manufacture of efficient solar cells.
The most frequently used method for producing quantum dots is Stranski-Krastanov epitaxial growth, which is based on a strained crystal lattice of the semiconductor growing on the substrate. As a result of this lattice strain, the growing layer does not grow uniformly. Instead, small nanometer-sized islands are formed, which constitute the quantum dots. Using this method, the size and density of the quantum dots can be controlled to a certain degree, while control of the arrangement and position is possible only to a very limited extent. Other methods for producing quantum dots use the methodology of scanning probe microscopy. These methods allow excellent control over the size and position of the quantum dots. However, they are sequential methods, in which each quantum dot must be produced individually. Therefore, such methods can be used only to a limited extent for devices having a large number of quantum dots.
The in-situ creation of quantum dots in a matrix is described, for example, in U.S. 2004/0092125 A1. There, a dielectric precursor is coated onto a thin metal layer on a substrate and gradually heated, whereby the metal layer and the precursor are gradually stacked on each other, so that quantum dots are formed from the precursor in the metal layer. U.S. Pat. No. 6,242,326 B1 describes a method for producing quantum dots, in which GaAs quantum dots are formed from Ga droplets and coated with a passivation layer which is formed of a buffer layer and a barrier layer. A similar method is described in KR 1020010054538A. Japanese document JP 2006080293 A describes a method of self-organized formation of InAs quantum dots on a GaAs layer, the quantum dots being embedded in a GaAs matrix. Further, it is described in U.S. Pat. No. 5,229,320 to deposit quantum dots through a porous GaAs membrane on an AlGaAs substrate, and to subsequently grow a matrix of AlGaAs for embedding purposes. A method for manufacturing a polymer containing dispersed nanoparticles is described in DE 601 08 913 T2. In that approach, first a polymer precursor is deposited, on which nanoparticles are subsequently distributed as quantum dots. The polymer is cross-linked by application of heat, thereby embedding the quantum dots into the matrix.
DE 694 11 945 T2 describes a method in which, first, a soluble precursor of a metal or a metal compound is dissolved in a vaporizable solvent. Then, the dissolved precursor is sprayed onto a substrate as finely distributed, nanometer-sized droplets. Thus, in this known method, the structure and distribution of the quantum dots are no longer dependent on the material and the substrate. The relatively severe limitations of the epitaxial growth method do not occur. The deposited nanostructured precursor is then brought into contact with a chalcogen-containing reagent, so that a chemical reaction occurs at room temperature to form quantum dots of a desired material composition comprised of the precursor and the reagent. The solvent may be vaporized before, during or after the chemical reaction. A polymer is additionally added to the solvent, and serves primarily to coat the dissolved precursor in the solvent and to prevent the nanoparticles from agglomerating during spray deposition. In addition, the polymer is deposited on the substrate, forming a matrix in which the quantum dots are embedded. A polymer matrix of this kind which is made of a transparent plastic has a certain refractive index for optical applications and may be stacked with other polymer layers of different refractive indices. The polymer is not subjected to a chemical reaction; it does not interact with the reagent. Materials other than a polymer cannot be used in the known method to form the matrix, because the matrix formation is merely a secondary effect, the matrix being formed on the substrate as a simple precipitate. The main purpose of the polymer used is to prevent the dissolved precursor particles from agglomerating, and therefore, must have corresponding materials and properties.
Document U.S. 2003/0129311 A1 describes a method which is similar but in which first a porous template is formed. The pores of the polymer are subsequently filled with a precursor solution from which the quantum dots are then formed.